Question 79: Why then doth Christ call the bread "his body", and the cup "his blood", or "the new covenant in his blood"; and Paul the "communion of body and blood of Christ"?

Answer: Christ speaks thus, not without great reason, namely, not only thereby to teach us, that as bread and wine support this temporal life, so his crucified body and shed blood are the true meat and drink, whereby our souls are fed to eternal life; (a) but more especially by these visible signs and pledges to assure us, that we are as really partakers of his true body and blood by the operation of the Holy Ghost as we receive by the mouths of our bodies these holy signs in remembrance of him; (b) and that all his sufferings and obedience are as certainly ours, as if we had in our own persons suffered and made satisfaction for our sins to God.

(a) John 6:51 I am the living bread which came down from heaven: if any man eat of this bread, he shall live for ever: and the bread that I will give is my flesh, which I will give for the life of the world.
John 6:55 For my flesh is meat indeed, and my blood is drink indeed.

(b) 1 Cor.10:16 The cup of blessing which we bless, is it not the communion of the blood of Christ? The bread which we break, is it not the communion of the body of Christ?
1 Cor.10:17 For we being many are one bread, and one body: for we are all partakers of that one bread.